Evolving possibilities: postembryonic axial elongation in salamanders with biphasic (Eurcyea cirrigera, Eurycea longicauda, Eurycea quadridigitata) and paedomorphic life cycles (Eurycea nana and Ambystoma mexicanum)

Abstract

Typically the number of vertebrae an organism will have post-embryonically is determined during embryogenesis via the development of paired somites. Our research investigates the phenomenon of post-embryonic vertebral addition in salamander tails. We describe body and tail growth, and patterns of postsacral vertebral addition and elongation in context with caudal morphology for four plethodontids (Eurycea), and one ambystomatid. Eurycea nana and A. mexicanum have paedomorphic life cycles; E. cirrigera, E. longicauda and E. quadridigitata are biphasic. Specimens were collected, borrowed and/or purchased, and cleared and stained for bone and cartilage. Data collected include snout-vent length (SVL), tail length (TL), vertebral counts and centrum lengths. Eurycea species with biphasic life cycles had TLs that surpassed SVL following metamorphosis. Tails in paedomorphic species elongated but rarely exceeded body length. Larger TLs were associated with more vertebrae and longer vertebrae in all species. We observed that rates of postsacral vertebral addition varied little amongst species. Regional variation along the tail becomes prominent following metamorphosis in biphasic developers. In all species vertebrae in the posterior one-half of the tail taper towards the tip. We suggest a developmental link might exist between the ability to continually add vertebrae and regeneration in salamanders.

Patterns of growth in the presacral vertebral column of the leopard gecko (Eublepharis macularius)

“…In summary, our null hypothesis, that all presacral vertebrae of the leopard gecko would grow isometrically relative to one another, and to overall body size, was rejected, as predicted. This outcome is consistent with similar findings published previously (Bergmann & Russell, ; Bergmann et al, ; Vaglia et al, ; Van Sittert et al, ). PC1, the overall proxy for body size, indicates that there is heterogeneity in scaling among the vertebral lengths when femur length is included in the analysis.…”

“…Following Bergmann and Russell (), we advance the null hypothesis that all presacral vertebrae of the leopard gecko grow isometrically relative to one another and to overall body size, thus retaining unchanging proportionality to each other throughout post‐hatching growth. While we have no a priori means of predicting particular patterns of allometric growth, previous findings for the entire vertebral column of the rat and the giraffe (Bergmann et al, ; Van Sittert et al, ), and the caudal series of other lizards and salamanders (Bergmann & Russell, ; Bergmann et al, ; Vaglia et al, ) suggest that allometric effects will be evident between individual vertebrae, vertebral regions, and vertebral series when assessed against non‐axial skeletal elements. We further hypothesize that the allometric growth displayed by the vertebrae will determine integration among them, expressed as patterns of covariance (Klingenberg ), either among the elements of the entire series or among the elements within compartments thereof.…”

“…Segmental growth, as represented by the vertebrae, has only been examined for the caudal series of a few salamanders (Babcock & Blais, 2001;Vaglia, White, & Case, 2012) and lizards (Bergmann & Russell, 2001;Bergmann et al, 2003;Bergmann et al, 2004), for the thoracolumbar vertebral column in four species of mammal (Jones & German, 2014), and for the entire vertebral column of two mammals, the Norway rat (Bergmann et al, 2006) and the Giraffe (Van Sittert, Skinner, & Mitchell, 2010). These studies have revealed differential growth rates between sequential vertebrae and vertebral series.…”

Body length shrinkage in an endangered amphibian is associated with drought

“…Although some differences in vertebral development exist between paedomorphic and biphasic Eurycea (e.g. timing of ossification of vertebral centra), vertebral morphology is nearly identical among species (Vaglia, White & Case, ). In addition, our examination of cleared‐and‐stained specimens of E. tonkawae revealed no intervertebral space within the spinal column.…”

‘Monster… -omics’: on segmentation, re-segmentation, and vertebrae formation in amphibians and other vertebrates

“…Furthermore, the final number of trunk vertebrae was not specified until late in development. This has also been shown for other salamander species that have the capacity to add segments to their tails (i.e., vertebrae) post-embryonically, at adult stages [33]. The number of embryonic somites, thus, would not specify the final number of vertebrae.…”